81x^4-72x^2y^2+16y^4
=(9x^2-4y^2)^2
=[(3x-2y)(3x+2y)]^2
=(3x-2y)^2(3x+2y)^2
(x-y)^2+4xy
=x^2+y^2-2xy+4xy
=x^2+y^2+2xy
=(x+y)^2